The present invention relates to a waste gas treatment system for treating a waste gas that is likely to generate dust when it is treated by combustion. For example, the present invention relates to a waste gas treatment system for treating hazardous and combustible gases containing silane gas (SiH4) or a halogen gas (NF3, CIF3, SF6, CHF3, C2F6, CF4, etc.) discharged from semiconductor manufacturing processes or liquid crystal panel manufacturing processes, or for treating scarcely decomposable gases.
Waste gases that are likely to generate dust include hazardous and combustible gases, e.g., silane (SiH4) and disilane (Si2H6) discharged from semiconductor manufacturing systems or liquid crystal panel manufacturing systems, and waste gases containing scarcely decomposable, global warming gases (PFCs). Such waste gases cannot be emitted into the atmosphere as they are. Therefore, the common practice is to introduce such waste gases into a pretreatment system where flames are formed in a furnace by using an auxiliary burning gas as a combustion gas, such as town (natural) gas or propane gas, and the waste gas is burned in the flames and thus made harmless by oxidation through combustion. Air is usually used as an oxidizing agent for the auxiliary burning gas.
When waste gas is treated at high temperature as stated above, fine particles (mainly SiO2) are generated as a by-product. To remove the fine particles, the treated gas is passed through a scrubber (reaction tower) such as that shown in FIG. 14. The scrubber 6 has a cylindrical casing I in which a pair of packing layers 2a and 2b are disposed a predetermined distance apart from each other, and a spray device 5 having a spray nozzle 4 at the distal end of a cleaning water piping 3 is installed between the packing layers 2a and 2b. With the scrubber 6 having the described arrangement, treated gas G passing through the inside thereof remains in the space between the packing layers 2a and 2b, where fine particles are removed with cleaning water sprayed from the spray device 5.
However, fine particles contained in the treated gas G have particle diameters distributed widely over a range of from several nm to several ten xcexcm. Although particles with large diameters can be captured, it has been difficult to remove fine particles not larger than 1 xcexcm. Accordingly, the rate of collection of fine particles has been confined to about 60 to 70%.
The present invention was made in view of the above-described circumstances, and an object of the present invention is to provide a waste gas treatment system designed to be capable of collecting, at a high collection rate and with high efficiency, fine particles contained in treated gas resulting from subjecting a waste gas to decomposition treatment at high temperature.
The present invention provides a waste gas treatment system including a primary cooling section provided at a stage subsequent to a decomposition treatment section in which a waste gas is subjected to decomposition treatment at high temperature, the primary cooling section having a liquid spray part for spraying a liquid on the treated gas from the decomposition treatment section; a secondary cooling section for cooling the treated gas sprayed with the liquid in the primary cooling section to reduce the volume of the treated gas; and a particle collecting section for injecting a liquid into the treated gas cooled in the secondary cooling section to collect fine particles contained in the treated gas. The particle collecting section uses a fan scrubber having a rotary impeller. With the waste gas treatment system arranged as stated above, the treated gas is cooled in the secondary cooling section to reduce the volume thereof, and fine particles contained in the treated gas are aggregated to flocs with large particle diameters. Then, a liquid is injected into the treated gas in the particle collecting section. Therefore, the flocs are captured efficiently. In addition, the volume of the waste gas to be treated is reduced. Accordingly, it is possible to increase the rate of removal of particles from the treated gas and to discharge clean treated gas.
In addition, a U trap is detachably provided in a drain pipe line for draining water containing foreign matter such as fine particles from the primary cooling section to collect the foreign matter. Thus, not only can the waste gas and waste liquid be separated from each other, but also blockage of the drain pipe line can be prevented. There is therefore no possibility that waste liquid containing harmful substances will overflow. When foreign matter has gathered in the U trap, it is only necessary to detach the U trap from the drain pipe line and to clean the inside of the U trap. Therefore, maintenance is greatly facilitated.
In addition, a sprinkling means for sprinkling water is provided in a waste gas pipe line through which treated gas from the decomposition treatment section flows. Thus, a flow of water can always be formed on the inner wall surface of the waste gas pipe line. Therefore, even if a treated gas containing a large amount of dust flows, there is no possibility of dust adhering to the inner wall surface. Even if the waste gas G contains a corrosive gas (e.g., HF gas), waste gas piping will not be corroded. Further, because water is sprinkled, even if a high-temperature treated waste gas flows, the high-temperature treated waste gas can be cooled with a minimal amount of water.
In addition, a mist catcher is provided in a waste gas pipe line through which the treated gas from the particle collecting section flows to catch mist contained in the treated gas. Thus, it is possible to prevent mist from flowing out of the system and hence possible to prevent corrosion of piping outside the system and to suppress an increase in pressure loss due to water droplets.
In addition, there is provided a waste gas treatment method wherein a waste gas containing dust and a water-soluble hazardous gas is introduced into a fan scrubber having a rotary impeller in which the dust and the water-soluble hazardous gas are adsorbed on cleaning water droplets to purify the waste gas, wherein the rotary impeller is rotated at high speed, i.e., not less than 55 m/s in peripheral velocity, and the amount of cleaning water used is maximized to promote mixing of water droplets and the waste gas in the casing of the fan scrubber, whereby the dust and the water-soluble hazardous gas are adsorbed on the cleaning water droplets and thus removed.
In addition, there is provided a waste gas treatment method wherein a waste gas containing hydrophobic dust and a hard-to-dissolve gas is introduced into a fan scrubber having a rotary impeller in which the hydrophobic dust and the hard-to-dissolve gas are adsorbed on cleaning water droplets and thus removed to purify the waste gas, wherein the rotary impeller is rotated at high speed, i.e., not less than 55 m/s in peripheral velocity, and the amount of cleaning water used is maximized to reduce the size of water droplets in the casing of the fan scrubber and to increase the density of the water droplets, thereby increasing the probability that dust particles not larger than 1 xcexcm, in which random motion such as the Brownian motion is dominant, will plunge directly into cleaning water droplets, and also increasing the area of contact between the hard-to-dissolve gas and cleaning water droplets, whereby the hydrophobic dust and the hard-to-dissolve gas are removed to purify the waste gas.
In addition, there is provided a waste gas treatment system wherein a waste gas containing dust and a water-soluble hazardous gas is introduced into a fan scrubber having a rotary impeller with a multiplicity of blades in which the dust and the water-soluble hazardous gas are adsorbed on cleaning water droplets to purify the waste gas, wherein the rotary impeller has a multiplicity of short blades on an outer peripheral portion thereof. Thus, it is possible to rotate the impeller at high speed without extra resistance and to increase the amount of cleaning water. Consequently, it becomes possible to reduce the size of cleaning water droplets in the casing and to increase the density of the water droplets and hence possible to remove dust and water-soluble hazardous gas from the waste gas with high efficiency.
In the above-described waste gas treatment system, the rotary impeller of the fan scrubber is provided with a multiplicity of minute holes in each blade. With this arrangement, it is possible to rotate the impeller at high speed without extra resistance and to increase the amount of cleaning water. In addition, minute droplets of cleaning water can be emitted through the minute holes of the blades. Consequently, it becomes possible to reduce the size of cleaning water droplets in the casing and to increase the density of the water droplets and hence possible to remove dust and water-soluble hazardous gas from the waste gas with high efficiency.
The rotary impeller may have a multiplicity of short blades on an outer peripheral portion thereof and further have a multiplicity of short blades and/or a multiplicity of obstacles on an inner peripheral portion thereof. With this arrangement, it is possible to remove dust and water-soluble hazardous gas from the waste gas with high efficiency as in the case of the above.